Here's the breakdown:
* Kami and Piccolo were originally one being. They were the "Namekian God" (Kami) and the "Namekian Demon" (Piccolo Daimao), two sides of the same being.
* They were separated by a ritual. This separation occurred centuries ago, and it resulted in Kami becoming the guardian of Earth and Piccolo Daimao becoming a powerful demon.
* They are not related by blood. They are two halves of a single entity that were forcibly separated.
So, while they were once one, they are now distinct beings with different origins and goals.